1. Field of the Invention
The present invention relates to a reference liquid-containing calibration cell for the calibration of various sensors which measure a gaseous or non-gaseous constituent parameter of a fluid.
2. Description of the Background Art
The use of various sensors to measure gaseous and non-gaseous constituent parameters of fluids has become common in recent years. Such sensors may include electrodes, optodes and the like, and are often used to measure constituent parameters of fluids such as biochemical fluids (e.g., blood), natural or sewage water, ferments, laboratory fluids, and the like.
Fluid constituent parameters which are frequently measured by such sensors include partial pressure of a gas constituent (e.g., pO.sub.2 and pCO.sub.2), pH, concentrations of various ions (e.g., sodium, potassium, calcium, chloride and the like), and concentrations of various organic molecules such as sugars (e.g., glucose), hormones and enzymes.
Prior to an accurate measurement of a fluid constituent parameter, often it is necessary to calibrate a sensor, including its associated instrumentation. Calibration techniques generally involve bringing a sensor into communication with a reference (calibration) liquid having a known concentration of the constituent to be measured.
A calibration fluid may be prepared for immediate use, however, such preparation is time-consuming and requires skill and accuracy. Once a calibration fluid is prepared, care must be taken to prevent interaction of the fluid with the surrounding environment which might alter its make-up and result in faulty calibrations. Such alterations in the make-up of a calibration fluid may result from permeation, diffusion, chemical reaction, and the like.
Because of the time and skill involved in the preparation of a reference fluid, and as a result of the problems associated with maintaining a reference fluid in usable condition over time, sensors are frequently calibrated using pre-packaged calibration fluids.
Two examples of packaging arrangements for reference solutions are described in U.S. Pat. No. 4,340,457 to Kater. One package includes a pair of electrodes for use as a potassium ion sensor, mounted in openings along the length of a cylinder containing a reference fluid. The cylinder has plastic caps at each end to seal the package. After calibration of the sensor, the caps are removed from the cylinder, the fluid is drained, and the cylinder may be inserted into an extracorporeal blood loop for measuring the potassium ion concentration of a patient's blood.
Another package described in the Kater patent includes a stoppered vial containing a reference fluid. A pair of electrodes for use as a potassium ion sensor are mounted in a catheter which is inserted through the stopper bringing the electrodes into communication with the fluid in the vial. Afater calibration of the sensor, the catheter may be removed from the vial and inserted into a blood vessel for measuring the potassium ion concentration of a patient's blood.
Because the packages disclosed in the Kater patent are either tightly capped or stoppered, they are not adaptable to calibration fluids for gaseous constituents. It is not practical to pass a reference gas into the calibration fluid contained in the packages described by Kater. The Kater packages are therefore limited to the calibration of sensors for ions or other fluid constituents present in the packaged reference fluid.
Another sealed package for calibration fluids is disclosed in U.S. Pat. No. 3,885,414 to Reville. The calibration fluid may contain pre-determined concentrations of dissolved reference gases (such as oxygen and carbon dioxide) as well as other reference constituents. The sealed package comprises a rigid hollow vessel of non-permeable material such as glass. At least one end of the vessel terminates at a sealed port. For calibration purposes, one end of the vessel is broken off, and the reference fluid is contacted with a sensor. This package has a disadvantage in that the package must be broken open prior to calibration of the sensor, which might result in fluid loss, contamination of the fluid or other detrimental interaction of the fluid with the environment. Another disadvantage is the possibility of chemical reaction between a gaseous reference constituent (particularly oxygen) and another reference constituent during storage of the sealed package, which could result in inaccurate calibration.
There thus remains a need for reference liquid-containing calibration packages or cells into which a reference constituent may be introduced without breaking open the cell and thus exposing the contents to the environment.